Elongated nanostructures such as nanocrystals, nanotubes and nanowires have become increasingly important due to their interesting electrical and optical characteristics. Nanostructures comprise both inorganic materials, such as silicon, germanium, and gallium nitride, and organic materials, such as carbon nanotubes and semiconducting polymers. In particular, single-crystal nanowires and carbon nanotubes have proven useful for high-mobility transistors on a variety of substrates.
Typically, these devices are made by spin-coating liquids containing elongated nanostructures onto a substrate. Electron-beam lithography may be used to pattern the circuits using standard methods and thereby remove elongated nanostructures from select regions. The remaining elongated nanostructures typically have random orientation which may be undesirable for some applications.
In order to control orientation, U.S. Pat. No. 6,872,645 entitled Methods of Positioning and/or Orienting Nanotructures by Duan et al. describes using microfluidic channels to control nanowire orientation. Although the microfluidic channels achieve some level of patterning, it has been difficult experimentally to achieve arbitrary patterns with good registration of the nanowires. Furthermore, building three dimensional microfluidic systems substantially increases the complexity of device fabrication.
Another disadvantage of current deposition systems is that substantial nanostructure material is wasted. Both spin coating and microfluidic channels use substantially more nanostructure material than is incorporated into the final device, especially when areas that need nanostructures are highly localized. Nanostructure material is expensive. Substantial wasted nanostructure material makes the cost of forming large area devices, such as displays, using the described techniques prohibitively expensive.
Thus an improved method of depositing and orienting elongated nanostructures is needed.